Communication devices such as wireless device are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been developed in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Recent developments of the 3GPP LTE facilitate accessing local Internet Protocol (IP) based services in the home, office, public hot spot or even outdoor environments. One of the important use cases for the local IP access and local connectivity involves the direct communication between devices in the close proximity, typically less than a few 10s of meters, but sometimes up to a few hundred meters, of each other.
This direct mode or Device-to-Device (D2D) enables a number of potential gains over the traditional cellular technique, because D2D wireless devices are much closer to one another than cellular wireless devices that have to communicate via cellular Access Point (AP), e.g., eNB:                Capacity gain: First, radio resources, e.g., Orthogonal Frequency Division Multiplexing (OFDM) resource blocks, between the D2D and cellular layers may be reused, i.e., reuse gain. Second, a D2D link uses a single hop between the transmitter and receiver points as opposed to the 2-hop link via a cellular AP, i.e., hop gain.        Peak rate gain: due to the proximity and potentially favorable propagation conditions, high peak rates may be achieved, i.e., proximity gain.        Latency gain: When the wireless devices, e.g., UEs, communicate over a direct link, eNB forwarding is short cut and the end-to-end latency may decrease.        
The Feasibility Study on Proximity-based Services, in 3GPP, feasibility study for Proximity Services (ProSe), has identified services that may be provided by the 3GPP system based on UEs being in proximity to each other. The identified areas comprise services related to commercial services and Public Safety (PS) that may be of interest to operators and users. PS may comprise, e.g., all first responders in case of an emergency such as police, firemen, etc. Commercial services may comprise any consumer application that is not a PS device. Commercial services may also be referred to herein as non-PS services. The objectives of this feasibility study are to evaluate LTE D2D proximity services, as indicated in Table 1, indicates the type of activity to be performed by a wireless device, and the the coverage condition in which the activity is performed by the wireless device, that is whether the wireless device is within or outside of network coverage.
TABLE 1OutsideWithin networknetworkcoveragecoverageDiscoveryNon public safety Public safety& public safetyonlyrequirementsDirectAt least publicPublic safetyCommunicationsafety requirementsonly
For a D2D system, message detection is a performance aspect, for which it is desirable to both increase the message detection probability and to reduce the false alarm probability.
Peer discovery may be the first step in the establishment of a D2D link, i.e., the devices discover the presence of their peer, which has partly similar functionality as the cell search procedure in LTE. Discovery may be made possible by one party sending a message signal, i.e., a discovery D2D message, and the other party scanning for such a message. By measuring the quality of the received message signal, estimation may be made whether the radio channel is good enough to establish a D2D link. Discovery messages may be transmitted periodically, carrying information about the identity of the transmitter UE. In the general procedure a neighbor wireless device is “discovered” once a message carrying the wireless device's identity is detected.
In more detail, the discovery procedure may be divided into different types in different dimensions. For example it may be divided into:
1. Open Discovery: where the Transmitter (Tx) wireless device may be discovered by all neighboring wireless devices in the proximity;
2. Restricted Discovery: where the Tx wireless device target at specific Receiver (Rx) wireless device(s), i.e., may only be discovered by specific Rx wireless device(s).
Thus, a discovery message, as used herein, may be understood as a message sent by a wireless device to be discovered by other radio network node/s, or to discover other radio network node/s, such as wireless device/s, over a D2D link.
The content of a discovery message, from a L1 perspective, is known as the payload of a discovery message. The payload is the sum of all the individual fields, each comprising a particular type of information, within the payload. The set of fields, i.e., payload fields, and corresponding length within the payload, is known as the payload format. The number of bits of the payload is known as the payload size.
The payload format may be different for different types of D2D discovery messages, as shown below.
1. For a non-public safety open discovery use case, the expected size of the information carried in discovery messages is currently assumed to be 192 bits, as shown in Table 2 below, which shows, for three different payload fields of information carried in discovery messages, the assessed length in number of bits.
TABLE 2PayloadAssessedFieldLengthProse160 bits ApplicationCodeProSe 8 bitsFunctionIDentifier (ID)PLMN ID24 bits
2. For a Public safety case, the message structure depicted in Table 3 is expected. Table 3, in addition to the columns shown in Table 2, shows the payload, the the assessed length and the purpose.
TABLE 3PayloadAssessedFieldLengthPurposeSource L2e.g. 48To identify a singleID/ProsebitsUE source of theUE ID ofinformation in thesourcemessage. This maybe used forsubsequentcommunication orto send a reply inModel B ofoperationDestinatione.g. 48To identify a singleL2 IDbitsUE or group of UEsthat are intendedrecipients of theinformation (asingle UE inresponses formodel B).Message 8 bitsType of discoverytypemessageProse64 bitsNeeded to performApplicationmatching to theIDrequired serviceSet/DiscoverycriterionUE mode 2 bitsDefines whether aofPublic safetyoperationProSe UE is actingas a UE-to-networkrelay, UE-to-UE orboth or not actingas relayPLMN ID24 bitsThe PLMN ID the(PublicProSe UE isLandattached to.MobileNetwork)Status bits 4 bitsSomestatus/maintenanceflags.
3. Discovery message for relay UE discovery where a UE is either announcing itself as a relay or requesting connectivity to a relay node. Such different relay discovery messages may be associated to different discovery message types or formats e.g. relay-related message formats.
The terms Public Safety (PS) and National Security and Public Safety (NSPS), as used herein, indicate public safety. The terms consumer and commercial, as used herein, are meant to indicate non-public safety applications.
If considering that the discovery message may have different payload sizes. It may cause ambiguity to the receiver (Rx) due to e.g.: a mix of consumer, public safety, relay UEs discoverable on the same carrier; or that the Rx does not know if a carrier is commercial, PS or acting as a relay.
One solution would be to let Layer 1 at the receiver decode all received messages depending on the different types of messages and based on different assumption of the message format. However, this would lead to higher complexity and an increase in computational burden on the receiver. Further, if e.g. the payload size is the same but the payload field format is different, the Layer 2/3 of the receiver would not know how to interpret the fields of the message, thus causing ambiguity at the receiver. Layer 1 (L1) is a physical layer, wherein L2 is a Medium Access Control (MAC) layer, and wherein L3 is a Radio Resource Control (RRC) layer and a Packet Data Convergence Protocol (PDCP) layer.